The present invention relates to a method for the radioactive decontamination of a steel surface, and particularly to a method for the radioactive decontamination of a steel surface using pickling. The invention relates, for example, to the radioactive decontamination of an internal circuit, a metal surface, piping or item of equipment in a reprocessing plant for irradiated nuclear fuel, hereinafter referred to as the xe2x80x9csurfacexe2x80x9d. The invention relates particularly to the radioactive decontamination of a surface made of austenitic steel, which is used to construct most surfaces of such plants.
The present invention also relates to a device for implementing the said method.
The radioactive contamination fixed on the surfaces of reprocessing plants is mainly due to surface adsorption. This contamination includes metastable radioactive contamination composed of Pu241, Am, U, 242, 244Cm, 137Cs, 90Sr and particulate radioactive contamination with ruthenium and the following insoluble compounds: cesium phosphomolybdate, zirconium phosphate, zirconium molybdate, plutonium phosphate mixed molybdate of zirconium and plutonium, oxides of Mo, Sb, Al, Fe, colloidal plutonium oxides, etc.
It is therefore unnecessary to cause significant erosion to decontaminate the surfaces. It is generally accepted that, depending on the surface, erosion of the order of between 2 and 10 xcexcm is sufficient.
Generally speaking, and particularly in the above example of decontamination of a reprocessing plant for irradiated nuclear fuel, radioactive decontamination may involve two major stages:
a first stage aimed at eliminating metastable contamination and the main deposits adhering to the surfaces, and
a second stage aimed at eliminating both particulate contamination fixed on the surfaces and residual deposits.
The first stage is a rinsing stage using a variety of sequences of rinsing that are non-corrosive for the surface; the second state is an erosive stage that uses reagents that are corrosive for the surface and mainly consist of oxidizing mixtures such as nitric acid/fluorhydric acid, nitric acid/cerium IV nitrate, or mixtures comprising chromic acid, nitric acid and cerium nitrate.
The nitric acid/fluorhydric acid mixture has the advantage of attacking refractory contamination deposits such as a variety of molybdates or phosphates such as Zr4+, MoO22+, Pu4+ and antimony oxides.
During the corrosion stage the best decontamination factors are generally achieved when erosion is both slow, i.e. at a kinetic rate of the order of 1 xcexcm/h or less, and regular. This is fairly difficult to achieve when the circuits to be decontaminated are complex and have zones that are more sensitive to, corrosion such as, for example, weld zones, zones of mechanical stress, etc.
Furthermore, the oxidant used for the corrosion must not be too harsh. This is why the oxidant mixture of nitric acid/fluorhydric acid cannot always be used as it is difficult to control, particularly on large extended surfaces.
Furthermore, some surfaces of the internal circuits of reprocessing plants for irradiated nuclear fuels are made of austenitic steel. It is therefore necessary on these surfaces to use an oxidant that does not cause intergranular corrosion of the steel. The oxidation-reduction couple of CeIV/CeIII is one of the rare oxidant agents that can be used to produce a given degree of erosion on austenitic steel surfaces without causing excessive intergranular corrosion.
However, the methods of the prior art using the above oxidation-reduction couple have the particular drawback of requiring, as a precondition of effective decontamination, large quantities of cerium (of the order 0.5 mole/m2/xcexcmxe2x88x921) and hence raising the price of effluent treatment as high concentrations of cerium are not authorized in the vitreous containment matrices. The concentration of cerium required in these methods is therefore a limiting factor.
In the methods of the prior art, the concentration of CeIV drops throughout the reaction, implying changing kinetics of corrosion: corrosion is too strong at the beginning and too weak at the end. This drawback is overcome by the present invention since the concentration of CeIV, and therefore the kinetics, is virtually constant.
In addition, the methods of the prior art, in particular the method described in the above-mentioned document, are designed to treat oxides and not the metal surfaces of valency 0 such as austenitic steel surfaces. The nature of the alloy to be eroded or corroded which, in a reprocessing plant is exclusively austenitic steel and not INCONEL(trademark) or INCOLOY(trademark) or similar means that the presence of chromic acid in the decontaminating solutions of the methods described for the latter is undesirable.
For example, patent application EP-A-0 174 317 discloses a method for decontaminating chrome oxides on the surface of INCONEL(trademark) steam generators in power plants. In this patent the CeIV used as an oxidation agent is used as an agent for regenerating Cerium IV from the Cerium III formed during oxidation. The method discloses the use as a corrosion fluid of an aqueous solution of nitric acid, chromic acid and Cerium nitrate in which ozone has been dissolved. It also uses a gas-liquid contactor to put the ozone into solution.
In the decontamination of a steam generator according to the method disclosed in EP-0 174 317, a regeneration chamber for Cerium IV is coupled with the steam generator, said chamber regenerating the Cerium IV by injecting ozone to saturation point. This cannot be envisaged in a reprocessing plant due to the high xcex1, xcex2 and xcex3 radiochemical activity of the components to be decontaminated and the great complexity and variety of components to be decontaminated.
Furthermore, the method disclosed in application EP-0 174 317 reduces without eliminating the variability in corrosion speed by virtue of its direct dependence on the value of the concentration of Cerium IV which changes during the course of the reaction since Cerium IV is regenerated in batches.
The present invention overcomes the drawbacks described above by providing a method for the decontamination of a steel surface consisting in bringing the surface to be decontaminated into contact with a pickling solution comprising nitric acid and a first iron oxidation agent at a suitable temperature such that the face of the said surface is eroded by the oxidation of the metallic constituents such as Fe0, Cr0, Ni0, Mn0 it contains, said bringing into contact being achieved by means of the direct, continuous introduction into the pickling solution of a gas comprising a second oxidation agent such as continuously to oxidize, at least partly, said first oxidation agent reduced by the oxidation of the metallic constituents of the steel.
During the course of the method of the invention the first agent for oxidizing the metallic constituents of the steel is reduced when it oxidizes the metallic constituents of the steel surface and it is continuously regenerated by the second oxidation agent, by oxidation. The first oxidation agent is therefore selected such as to be capable of oxidizing the metallic constituents of the steel surface and the second oxidation agent is selected as being capable of oxidizing the first oxidation agent reduced by oxidation of the metallic constituents of the steel to regenerate it.
The first oxidation agent may, for example, be selected from CeIV, Ag2+, etc.
The gas constituting the second oxidation agent may be selected from gases comprising ozone.
Advantageously, according to one embodiment of the method of the present invention, the first oxidizing agent may be cerium IV, for example as cerium IV nitrate, and the second oxidizing agent may be a gas including ozone.
According to this embodiment, the present invention makes it possible in particular to eliminate the above drawbacks relating in particular to the methods of the prior art using cerium.
The present invention proposes means for continuously regenerating cerium IV throughout the reaction at a constant speed. Throughout the reaction the concentration of cerium IV is thus constantly at the optimal value for decontamination. The present invention discloses a method by which a second oxidizing agent, for example ozone, is dissolved in the pickling solution (also referred to below as the decontamination solution) directly in the component to be decontaminated. Furthermore, the method disclosed in the present invention makes it possible to solubilize the second oxidizing agent, for example ozone, in the pickling or decontamination solution without making any modification or adding any special fittings to the installation to be decontaminated.
According to the invention the pickling solution may be an aqueous solution containing nitric acid at a concentration of approximately 0.5 to 5 mol.1xe2x88x921 and cerium nitrate at a concentration of approximately 0.001 to 0.1 mol.1xe2x88x921.
According to the invention the gas including ozone may also include at least one gas selected from either oxygen and nitrogen.
According to the invention the gas including ozone may include approximately 1 to 20% ozone.
According to the invention the appropriate temperature may be approximately 10 to 80xc2x0 C., for example ambient temperature, for example 25xc2x0 C.
According to the invention the steel surface may be a surface of an internal circuit of a reprocessing plant for irradiated nuclear fuel, for example a surface made of austenitic steel.
The ozone introduced into the pickling solution may, for example, be dissolved in the solution by using two modifications of a component that is very common in reprocessing installations and usually used to initiate movement in liquid solutions. In the application of the method of the present invention the introduction of ozone into the erosion solution may be achieved using two gas-liquid contact components that may either be transfer air-lifts, of either the naturally submerging or vacuum submerging type, and/or air-lift agitators. However, in the absence of these two systems the invention may be embodied by injecting gas by means of any type of submersible piping for introducing a liquid or gaseous reagent. By virtue of their use as disclosed in the present invention the said devices play a dual role during decontamination:
by performing the agitating function they ensure continuous renewal of the liquid-solid interface at which the oxidation stages of the metal support and dissolution of oxidized species occur,
by providing close gas-liquid contact they ensure transfer of the second oxidizing agent, for example ozone, into the pickling solution at the threshold of which the regeneration stage of cerium IV occurs.
According to the method of the invention, by introducing or injecting gas containing the second oxidizing agent, for example ozone, via the two air-lift systems into the decontamination solution, or pickling solution, the gas, for example ozone, plays a dual role:
it is used as a power source for hydrodynamic agitation,
it adds the second oxidizing agent, for example ozone, which is necessary for the regeneration of cerium IV, to the solution.
The present invention also provides a device for the radioactive decontamination of a steel surface of an internal circuit of a reprocessing plant for irradiated nuclear fuel according to the method of the present invention, said internal circuit being provided with an agitator and/or transfer air-lift, a device, in which the air-lift is used both as a means for introducing the gas containing the second oxidizing agent into the pickling solution containing the first oxidizing agent, and as a means for homogenizing the pickling solution when the said solution is brought into contact with the steel surface to be decontaminated.
According to the invention the device may, also include an assembly for producing the second oxidizing agent, for example when the first oxidizing agent is cerium IV the second oxidizing agent may be ozone and the said production assembly may therefore be an assembly for producing ozone. This assembly may be a standard type of assembly known to those skilled in the art for the production of ozone.
The fact that the present invention uses two different types of air-lift, the method makes it possible to select accurately and simply what portion of an installation is to be decontaminated.